This would be a pretty heavily stocked tank but so long as you keep up with water changes, you should be just fine. The cories will roam around the bottom, the apistos will cruise around down there, too, the cardinals will stay in the mid levels and the hatchets will stay near the surface. Plant the heck out of it, get enough filtration to turn it over 10 or so time per hour, and you'll be all set.

Def check for used tanks, just be sure before you buy one to take a good look at the silicon seals that they're not damaged.
For the corys i'd def also suggest 8, since the room in the tank on the bottom would be big enough.
Now on the serpae tetra, if you really want them: You gotta know up front they're fin nippers and they pretty much go after any other fish (almost everyone) so that may not be an ideal choice if you wanna add other fish later down the Road.
Unless you really really LOVE pleco's and HAVE TO have one, i'd not get one for a 29g tank, that'd just add to your bio load for no reason at all then.

If you get any tank between 60-120 liters you can do something with it. Even if you were to only get a 20-30 liters for now, some plants and some Killifish like"Norman's Lampeye" these very Killifish you could easly house a group of 15-20 in a small tank like that.

Just keep in mind while making your size decision that a smaller tank is harder to maintain then a larger tank (as far as good water quality for fish)

~ Life Is Too Short, Break The Rules, Forgive Quickly, Kiss Slowly, Love Truly, Laugh Uncontrollably And Never Regret Anything that Made You Smile.
Life May Not Be The Party We Hoped For, But While We're Here, We Should Dance. ~

Well you could leave them out entirely, of course. Other smaller plecos include clown and rubberlip plecos.

I suggest the hatchets because...well, there aren't a whole lot of options for top-dwelling fish for an Amazonian tank. Things like gouramis, danios and some rasboras work well but these fish aren't from the area so they wouldn't fit in with the theme.

There may be other species of tetra that tend to hang out near the surface, but I'm not familiar with them.

I'd have to say a 35 L (9g) is out of question then with this stock list, you'll need at LEAST a 60 L (15g) and even that's cutting it quite short with the blue rams and school Tetra (I'd then suggest Tetra that stay small)

~ Life Is Too Short, Break The Rules, Forgive Quickly, Kiss Slowly, Love Truly, Laugh Uncontrollably And Never Regret Anything that Made You Smile.
Life May Not Be The Party We Hoped For, But While We're Here, We Should Dance. ~

You also need to think about water parameters. Some fish can adapt (to a certain extent) and others can't (like the blue rams you mention) so it is a good idea to know your water parameters out of the tap and think of fish that will match those. This is easier than trying to fiddle with parameters if your water is extreme. It may not be, but as you're in the UK I seem to recall many there have basic (alkaline) water somewhat on the hard side. Also think about temperature, not all fish will live healthily in water that is warmer or cooler than their preference. Again thinking of the rams, they are warm water (82F minimum) and some tetras, such as neons for example, would literally burn out long-term at that high a temperature; most (but not all) corydoras would also.

As you seem to be thinking South America, have a look at the photos of my Amazonian Riverscape 115g and flooded Amazon forest 90g aquascapes. I am a geographic aquarist, everything (plants, fish, items like wood or rock) in each tank is from or can be found in watercourses in the same geographic area, such as the Amazon basin, though not the same stream which would be a true biotope. The list of fish and plants in these tanks may give you some ideas. Many of my fish are wild caught and very specific regarding water parameters. I and others can offer suggestions in this area.

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian.[unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]